The present invention relates to a system and a method respectively for dispersion compensation in a fibre optic high speed communication system. Fibre optic transmission systems or more particularly fibre optic high speed systems are being used in communication in various ways, as for instance in telecommunication and over long transmission distances. Certain networks, as for instance national networks, demand very high transmission speeds, often 2.5 Gbit/s but the transmission speed may also be considerably higher, for instance 10 Gbit/s and more. The transmission speed is however limited by the occurance of dispersion between transmitter and receiver. The dispersion starts to give rise to problems at approximately 2.5 Gbit/s. At 10 Gbit/s it gives rise to a considerable limitation due to the fact that the information band width itself gives rise to dispersion. Problems with dispersion thus occur at high speeds and long distances. In ordinary single-mode fibres the significance of dispersion increases at bit rates higher than 2.5 Gbit/s for wavelengths of approximately 1.55 .mu.m. At often utilized repeater-distances of approximately 60 kilometers, the effect is significant at 10 Gbit/s. Thus, it is of great importance that the dispersion is limited as much as possible, in order to achieve as long transmission distances as possible.
A number of devices and methods, respectively, for compensation of the dispersion in fibre optic transmission systems, in particular high speed systems, have been proposed.
A number of known solutions are based on so called prechirp technology. This is based on the fact that the frequency/wavelength of the laser is modulated during each pulse. The frequency spectrum is distorted in such a way that the pulse in the fibre converges. Both directly and externally modulated systems are known, and usually the laser achieves the prechirp effect in both cases. An example of such a device is described for instance in "Dispersion Compensation by Active Predistorted Signal Synthesis", T. L. Koch, R. C. Alferness, J. of Lightwave Technology, Vol. LT-3, No. 4, (1985), pages 800-805. At 1.05 .mu.m and for an ordinary single-mode fibre it is required that the signal should blue-shift. By direct modulation, a laser normally red-shifts during the pulse. Usually the laser is FM modulated for obtaining the chirp, after which the AM modulation is applied by means of an external modulator. In for instance "10 Gb/s 100-km normal fiber transmission experiment employing a modified prechirp technique", N. Henmi, T. Saito, M. Yagamushi, S. Fujita, Proc: OFC'91, (1991 ), paper Tu02, it is described how selected DFB lasers are utilized. With a so called blue-shift modulation in the transmitter, see for instance "Frequency Chirping in External Modulators", F. Koyoma, K. Iga, J. of Lightwave Technology, Vol. LT-6, No. 1, (1988), pages 87-93, the FM/AM modulated signal is obtained in an external modulator whereby the laser functions without any influence. In order to obtain the required modulation it is normally required that the external modulator be designed in a special way. Both in the case where the laser carries out the prechirp function and where an external modulator carries out the prechirp function, there is a wish to obtain a blue-shifted pulse. Both the prechirp generation and the blue-shift modulation in the transmitter utilize the dispersion in order to achieve a pulse compression.
By means of another known device, the transmission is made dispersion-free by adding an additional length of fibre which has a reversed sign of the dispersion, this is described for instance in "Multiwavelength Dispersion Compensation for 1550 nm Transmission at 2,5 Gb/s Over 1310 nm Optimized Single-Mode Fiber", H. Izadpanah, C. Lin, K. Runge, M. Z. Iqbal, J. L. Gimlett, Proc: ECOC'92, (1992), paper TuA5.1. A device where an already laid-out fibre is utilized means that a dispersion-compensating fibre may be arranged before the receiver. The compensating fibre length can be approximately a third of the transmission distance. This gives a number of disadvantages due to the fact that the extra fibre or fibre length is expensive, it demands a special design and it also adds attenuation. In the above-mentioned document the losses are limited by arranging a fibre amplifier between the transmission fibre and the dispersion compensation fibre, which further complicates the system and makes it more expensive.
A number of devices are also known by which the dispersion compensation of the signal is carried out on the receiving side of the transmission system. An example is described in "Microstrip Compensation of Fibre Chromatic Dispersion in Optically Amplified Coherent Systems", J. J. O'Reilly, M. S. Chauldry, Proc: EII Colloquium on Microwave Optoelectronics, No 139, (1990), pages 13/1-13/6. This device is based on phase compensation of the frequency spectrum of the received signal, which counteracts the phase difference which the different partial frequencies have received in the laid-out fibre. This device, as well as others of the same kind, requires the use of heterodyne technology in the receiver. However, this is a complex and expensive technique. In the document described, the phase distortion is treated at an intermediate frequency. A mixer is arranged which consists of an optic direction coupler which is fed by a signal and local oscillator, a detector diode and a bandpass filter, and which only lets through the difference frequency. The phase correcting element is formed by a microstrip conductor having a normal dispersion. The microstrip conductor can for instance be 10-20 cm long and compensates for the dispersion in a fibre of a couple of hundred kilometers. After that, the electric signal is detected in a normal way.
European Patent Application No. 0 256 809 describes a device for dispersion compensation which is based on a multi-mode structure as dispersion compensating element. The signal is divided in a number of partial wavelengths which can then propagate over distances of equal lengths but with different group velocity. For digital communication systems in the Gbit/s range, the relative delay time may be in the order of 100 ps. A time difference of this size is hard to carry out with concepts which are based on the fact that the partial wavelengths shall travel the same distance but with different group velocities, the losses thereby giving rise to serious problems. Furthermore, a device of this sort is not flexible.
European Patent Application 0464812 describes a device in which a fibre is made dispersion-free by joining a number of elements having opposite signs of the dispersion for a given wavelength. The signal is split into several partial wavelengths which may propagate different distances in space, and through the use of a lattice a number of wavelengths are obtained. This device is based on so called "free-space"-communication which means that no waveguides exist and the difference in distance should be approximately 3 cm in order to achieve a time difference of approximately 100 ps. Even if the difference in distance may itself be obtained through the device, it will become sensitive to interference and badly suited for large-scale production. Furthermore, the device suffers from a lack of stability.